a population of 216 specimens of stenocereus eichlamii (cactaceae,subfamily cereoideae) was surveyed for triatoma ryckmani (vector of chagas disease)in a one hectare plot of semiarid habitat in guatemala. out of 44 plants that had dead and dry sections,24 plants had a total of 103 specimens of t.ryckmani .in comparison with other areas of guatemala,t.ryckmani is well established in the semiarid ecosystem (infestation index 54.5,density =2.3 and crowding index 4.3).the insects were mainly found (52.4%)in the dead portions of s.eichlamii 2.0 to 3.2 m above the ground (x2 =26.0,p<0.00001),followed by dry cactus sections between 3.2 and 5.0 m (35.9%).they were less frequent 0.2 to 2.0 m above the ground.a considerable proportion (75.7%)had no aparent blood in their digestive systems.to determine the presence of flagellates,43 of the bugs were dissected,but none were found.this is the first report on t.ryckmani population dynamics in this habitat. rev. biol. trop. 52(4):931-936.epub 2005 jun 24.